chore(tabs): remove unnecessary code

Author: adrianptrv

projects/igniteui-angular/src/lib/core/styles/components/tabs/_tabs-theme.scss

@@ -336,8 +336,6 @@
         @if $bootstrap-theme {
             border: none;
             box-shadow: inset 0 0 0 rem(2px) var-get($theme, 'item-active-hover-color');
-            border-top-left-radius: var-get($theme, 'border-radius');
-            border-top-right-radius: var-get($theme, 'border-radius');
             border-bottom-left-radius: rem(4px);
             border-bottom-right-radius: rem(4px);
             z-index: 1;